Evaluation of high density DRAMs as a nuclear radiation detector

Appl. Radiat. lsot. Vol. 48, No. 10-12, pp. 1601-1604, 1997

Pergamon P I h S0969-8043(97)00160-7

© 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0969-8043/97 $17.00+ 0.00

Evaluation of High Density D R A M s as a Nuclear Radiation Detector H. P. C H O U ,

T. C. C H O U

a n d T. H. H A U

Department of Nuclear Engineering, National Tsing Hua University, Hsinchu 300, Taiwan The research is based on the nuclear radiation induced soft error phenomenon associated with dynamic random access memory devices (DRAMs). Samples of 256 kbit and 1 Mbit decapped DRAMs from several manufacturers were exposed to standard alpha sources and showed a linear response with an intrinsic detection efficiency approaching 10%. Sensitivity studies were performed to evaluate the effects of DRAM operating voltage, refresh frequency and the data pattern stored prior to irradiation. The associated mechanism of soft error phenomenon is discussed. Samples were also exposed to gamma rays up to 105 rad to examine the total dose effect. The annealing phenomenon after gamma exposure is also presented. © 1997 Elsevier Science Ltd. All rights reserved

Introduction A radiation damage mechanism associated with dynamic random access memory devices ( D R A M s ) is called soft error, which flips the content of the memory cell but causes no damage to the electrical circuits; the error can be corrected by rewriting the memory cell (Yaney et al., 1979). This phenomenon has motivated us to use D R A M s for nuclear radiation detection, particularly for charged particle detection or neutron detection by coating boron or gadolinium on the memory cells. The basic counting procedure is quite simple: first reset the memory contents, expose to radiation, and then count the number of flipped cells; once counted, reset the memory contents and repeat the procedure. The use of D R A M s as a radiation detector is different from that of conventional nuclear radiation detection systems. The operating voltage of the detector is low, 5 V. The D R A M has a built-in charge amplifier and a charge comparator/discriminator circuit. Output information is expressed as digital numbers and no external signal processing instruments are required. The data read/write circuit is the same as the memory circuit used in personal computers, an off-the-shelf product, and thus is inexpensive. Furthermore, memory cells are arranged in a two-dimensional array structure, with potential for imaging purposes. Recent advances in semiconductor technology have pushed the D R A M s to a higher cell density; the physical dimension of a memory cell is smaller and the amount of charge needed to flip its contents is also less (Haque et al., 1986). In other words, a higher density D R A M tends to be more sensitive to radiation and is a more promising candidate as a

detector. We therefore chose commercially available 256 kbit and 1 Mbit D R A M s for the present study to investigate their characteristics as a radiation detector. We first present the results obtained from samples irradiated by alpha particles and discuss the sensitivity of D R A M s ' operating parameters. G a m m a doses up to 105 rad were applied; in contrast to charged particles, gamma exposure provides a total dose effect on D R A M samples. Section 4 presents the associated soft error phenomenon and also the annealing effect.

Alpha Radiation Effects Samples of 256 kbit and 1 Mbit D R A M s obtained from four manufacturers (Table 1) were used for the present study (Chou, 1994). Figure 1 illustrates the experimental apparatus. Samples were first decapped and inserted into a socket which is wire-linked to an interface circuit board. The interface circuit board is used for data transfer between the tested D R A M sample and the personal computer. The interface circuit board also provides connections to a power supply and a signal generator to adjust the D R A M sample's operating voltage and refresh frequency during testing. Soft-error counting is software-controlled: first reset the memory contents and then count the number of flipped cells at a preset time interval; once counted, reset the memory contents. If the memory cell cannot be reset to the original content, we consider it to be a hard error. Americium-241 alpha sources with an active diameter of 5 mm and calibrated activities of 0.1, 0.5, and 1.0 ~Ci were used. Irradiation results (Fig. 2) indicate that the soft error rate is linearly related to

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Table 1. DRAM samples used for testing Memory size ( b i t s ) Manufacturer Model No. 1M Hyundai HY531000 1M Panasonic MN41C1000 1M Toshiba TC511000 1M Samsung KM41C1000 256 k Samsung KM41256 256 k Mitsubishi M5M4256 256 k NEC D41256C 256 k Hitachi HM50256

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Previous studies indicated that D R A M operating conditions do affect the radiation induced soft error phenomenon (Darambara et al., 1993). Operation parameters that we evaluated in the present study were operating voltage, refresh frequency, and the data pattern initially stored in D R A M samples prior to irradiation. Figure 3 shows that the soft error rate decreases with increasing operating voltage; in other words, the higher operating voltage gives a larger amount of charge for a fixed capacitance of each

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TIME (rain) Fig. 2. The number of soft errors vs irradiation time for an 1 Mbit DRAM irradiated by a 1 laCi alpha source. memory cell and thus provides a higher charge threshold for the incoming radiation to upset the memory cells; similar results were observed in the literature (Darambara et al., 1993). Figure 4 shows the relation between soft error rate and refresh frequency; the sample was irradiated using the 1 ~tCi alpha source for 10 min. Results indicated that a higher refresh frequency causes a higher soft error rate; a similar phenomenon was also observed in the literature when D R A M samples with lower cell densities were used (Carter and Wilkins, 1985). According to Carter and Wilkins' study, the soft error observed in D R A M s may be classified as the cell mode and the bit-line sensing mode. The cell mode is a phenomenon of charge upset in the memory cell; it is similar to a charge perturbation in a capacitor and the data can only be flipped in one direction. The bit-line sensing mode is due to the radiation induced charge perturbation on the sensing amplifier and the data can be flipped in either direction; i.e. from 0 to 1 or from 1 to 0. With increasing refresh frequency, the sensing amplifier works in a higher frequency to restore charge and is thus more volatile to radiation damage as observed.

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10 -irradiation time as well as the source intensity. In other words, the preliminary results indicated that the D R A M has the basic property to serve as a counter. Samples from other manufacturers also showed a linear response; however, the sensitivity varied significantly among manufacturers. Experiments showed that samples of 1 Mbit D R A M do not necessarily have a higher soft error rate than samples of 256 kbit D R A M . This variation is attributed to the different design in the manufacturing process. D R A M samples from the same manufacturer also showed a rather large variation in sensitivity; chemicals used in the decap process may have caused damage on some samples. We thus only selected D R A M samples that were more sensitive to radiation in the following experiments. The selected samples were tested over a period of several months and showed stable results with the counting fluctuations near a Poisson distribution.

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Fig. 5 The number of memory cell errors in a 1 Mbit DRAM irradiated at a gamma dose rate of 16 krad/h.

Therefore, soft errors introduced by higher refresh frequency belong to bit-line sensing mode. In contrast, the effect o f the operating voltage o n the soft error is different. As stated before, the operating voltage affects the a m o u n t o f charge stored in the m e m o r y cell. Thus, soft errors introduced by lower operating voltage belong to the cell mode. Also s h o w n in Fig. 4 is the dependence of the initial d a t a pattern; patterns P1 a n d P0, representing the stored data, are all ones a n d all zeros, respectively. Results indicated that the all-one pattern has a higher soft error rate. A t the same time, we tested the z e r o - o n e c h e c k e r b o a r d p a t t e r n and surprisingly f o u n d the n u m b e r o f errors was smaller t h a n those o b t a i n e d from P0 a n d P1 patterns. Previous studies (Wyatt et al., 1979) measured radiation-induced soft errors using 4 kbit D R A M samples with a pattern immediately after the power was turned on. W e also

observed the effect o f the initial data pattern using the p o w e r - o n pattern a n d its c o m p l e m e n t a r y pattern. F o r the particular test sample (Hyundai 1 Mbit), the p o w e r - o n pattern is similar to a checkerboard pattern. Results showed that the power-on data p a t t e r n gave almost n o soft errors; in contrast, its c o m p l e m e n t a r y p a t t e r n gave the highest n u m b e r of errors. The results suggest that the D R A M sample with the power-on data pattern is in the most stable condition. The results also explained that the z e r o - o n e checkerboard pattern has less soft errors because of the similarity to the power-on pattern. To analyse the soft error m e c h a n i s m associated with the data pattern, we observed the trend s h o w n in Fig. 4 that the two curves for P0 a n d P1 patterns are almost parallel. In other words, the slope is independent of data pattern. This p h e n o m e n o n indicates the initial data p a t t e r n only affects the a m o u n t of soft errors that are introduced by the cell mode.

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According to the sensitivity study of the operating parameters, we concluded that decapped DRAM samples operated using a lower voltage, a higher refresh frequency, and having a complementary power-on initial pattern would have more soft errors under alpha irradiation. Using the Hyundai 1 Mbit DRAM samples, we found that operating under 5 V using a refreshing frequency of 1000 Hz and a complementary power-on pattern can generate a soft error rate of approximately 1000 counts/s by a 1 gCi alpha source; it corresponds to an intrinsic efficiency of approximately 10%.

Gamma Radiation Effects Samples were also exposed using a cobalt-60 gamma source at locations having dose rates in the range 16-256 krad/h with the total dose up to about 105 rad. In contrast to the alpha irradiation, there was no apparent relation between the number of errors and the gamma radiation intensity. Figure 5 is a typical gamma response curve; the number of soft errors rises rapidly at a certain threshold dose. At a higher total dose, soft errors decrease while hard errors increase; the total number of errors is then approaching to a steady value. All samples tested showed the same phenomenon; the threshold dose is not sensitive to the initial data pattern stored in the DRAMs but varies with manufacturer due to different chip designs. It is difficult to judge whether the induced error mechanism is a cell or a bit-line sensing mode; during the transition stage memory cells experience a very unstable condition. In our opinion, the large amount of soft errors which rapidly turned into hard errors is more likely an indication of device failure. In viewing the nonlinear gamma response of DRAMs, DRAMs are unlikely to be used as a gamma counter in usual applications. The relatively high gamma dose threshold, however, does suggest that DRAM samples are basically free from gamma background interference in measuring other radiations. Although a high gamma dose caused a significant number of errors in a very short time, the number of errors started to decrease after irradiation. The phenomenon suggests that the radiation-induced charge traps are near the interface of oxide layers and there is no permanent structure damage in DRAMs. As shown in Fig. 6, the DRAM sample, kept at room temperature, had completely recovered 6 months after irradiation.

Summary High density DRAM samples were tested using alpha and gamma sources. The soft error response to alpha charged particles showed a good linear behaviour and can serve as a particle counter. Detection sensitivity, however, depends on the chip design used by the manufacturer. Sensitivity studies on DRAM operating parameters showed that a lower operating voltage and a higher refresh frequency will give a higher detection efficiency. Soft error rate is also sensitive to the data pattern stored prior to irradiation; the power-on pattern is the most stable and its complementary pattern is the most sensitive to alpha particles. The most sensitive sample can give an intrinsic detection efficiency of 10%. Gamma irradiation tests showed that the number of memory errors increased rapidly at a certain threshold dose; the relatively high gamma dose threshold suggests that DRAM samples are free from gamma background interference in measuring other radiation fields. Experiments also indicated that DRAM samples suffer no permanent damage; the errors can be recovered slowly. In viewing its easy setup, proper detection performance, and efficiency, the high density DRAM is a promising counter for charged particle detection. If coated with neutron sensitive material, DRAMs may also be used for neutron detection. work was supported by the National Science Council, Taiwan under the contract No. NSC 84-2212-E007-021 and NSC 85-2212-E007-090.Assistance in the experiments and comments from Dr. T. H. Tseng, Institute of Nuclear Energy Research, Taiwan are acknowledged. Acknowledgements--This

References Carter P. M. and Wilkins B. R. (1985) Alpha particle induced failure modes in dynamic RAMs. Electronics Letters 21, 38-39. Chou, T. C. (1994) Evaluation of DRAM soft error mechanism for nuclear radiation detection. Master Thesis, Department of Nuclear Engineering, National Tsing Hua University, Taiwan. Darambara D. G., Beach A. C. and Spyrou N. M. (1993) Development of a novel neutron detector for imaging and analysis. Journal of Radioanalytical and Nuclear Chemistry' Articles 167, 197-208. Haque A. K., Yates J. and Stevens D. (1986) Soft error rates in 64k and 256k DRAMs. Electronics Letters 22, t188-1189. Wyatt R. C., McNulty P. J. and Toumbas P. (1979) Soft errors induced by energetic protons. IEEE Transactions on Nuclear Science NS-26, 4905-4910. Yaney D. S., Nelson J. T. and Vanskike L. L. (1979) Alpha particle tracks in silicon and their effect on dynamic MOS RAM reliability. IEEE Transactions on Electron Devices 26, 10-16.